I just made about 70 huge heart sugar cookies yesterday for a Valentine's party my little guy went to today. The icing I used came out perfectly. It dries hard and glossy and takes food coloring (liquid or paste) very well. The recipe is:
1 cup powdered sugar
1 tsp. almond extract (or vanilla if you prefer)
3 tsp. milk
3 tsp. light corn syrup
You can add more milk and/or corn syrup to get the desired consistency. After stirring all ingredients together until smooth, add small amount of coloring if desired and spread or pipe onto cookies. It takes about 90 minutes for the icing to dry hard.